Or it could just be using ca65 & ld65 in the back end, instead of implementing their own assembler & linker.

_________________
WFDis Interactive 6502 Disassembler
AcheronVM: A Reconfigurable 16-bit Virtual CPU for the 6502 Microprocessor

I'm speculating, but as far as I understand, gcc is built in three stages (that's why it takes so long to build, too).
1. A different, existing compiler (in this case cc65) is used to compile (some of?) the gcc source code
2. This results in an executable gcc which is then used to compile itself.
3. The output of stage 1 and stage 2 are compared.

===Jac

Just to note, the good people at Stardot have picked up this tool - maybe some more information or even patches will come out of that:
http://stardot.org.uk/forums/viewtopic.php?f=2&t=13148

(And I'd entirely forgotten about the tool, this thread, and my own actions as reported above. Within a year.)

There's a new thread and a new getting-started guide for this GCC by 'flibble' over on Stardot and therefore somewhat specifically for Acorn's BBC Micro - but doubtless more generally applicable:
Itszor/puppeh gcc cross compiler for the BBC: Instructions, examples and library

(This thread and the document linked above refer, I think, to the m65x-gcc6-virt branch, but it may be of interest that there's also an m65x-gcc8 branch.)

Edit: note that the repo has moved from one username to another so there are links around which no longer work. Substitute itszor for puppeh as appropriate.